This project was to investigate the efficacy of the GS-34 triode for use
as an amplifier at 902MHz. John N8OU had already attempted to use it at 23cm and found
the thermal instability to render it unsuitable, and although it was strongly suspected that
the 33cm band would also be a problem, there was enough doubt to check it out. The PA shown
in the pictures above was constructed, knowing that if it did not work, the work would not be
totally wasted because it could be converted to use the GS-15B by adding a few more sleeves
and a decoupling capacitor. This project showed, yet again, that triodes are not suited for
use at higher frequencies which use hi-Q, narrow bandwidth tuned circuits.

33cm Performance:

Initial tests showed excellent gain, with 10W drive and 2300V on the plate producing 500W out.
Input return loss exceeded 20dB. Output tuning had a slight effect on input tuning due to
imperfect isolation between input and output as is expected with triode PAs. The problem is
that any keydown period exceeding 3-5 seconds started thermal run-away. Plate current
increased rapidly and output dropped. This means that the PA could be useful for SSB and CW
only. Water cooling did not improve performance as compared to air cooling. This indicates
that the problem is, as with all triodes at frequencies above 432MHz (and even some there),
related to distortion of the grid causing capacitance changes internal to the tube. At these
higher frequencies, very small capacitance changes within a tube make large differences in the
tuned frequency of high-Q circuits such as cavities, making it impossible to keep the PA tuned
at power levels approaching the actual capability of the tube. The GI-7B, for example, is
capable of over 400W out on 23cm, but cannot be run over 200 to 250W output without severe PA
detuning.

In this case, with 1600V on the plate and 8W drive, 200W out for over 60
seconds was maintained. Again, whether air or water cooled made no difference. At this
point, attempts to use the GS-34 triode were suspended, and conversion to the GS-15B tetrode
begun. Pictures of the various pieces showing construction for those readers who can live
with the limitations of this tube on this frequency are included. Dimensions can be determined
fairly well from the drawing which IS to scale - follow instructions at the drawing download
links, below.

Click on picture to enlarge

Single GS-34-1 PA

Download a "to-scale" drawing and take appropriate measurments
using the two given dimensions as reference; on SOME computer systems, this drawing prints out
1:1, on others it prints out about 3/4 scale. View#1View#2 Print using your
browser's print function.

Pictures (Click to zoom):

Anode Cavity Ring

Anode Cavity Top

Anode Capacitor

Anode Fingerstock

Anode Insulators

Anode Capacitance

Anode Cavity Bottom Plate

Bottom Plate fingerstock

Input Resonator

Input Cavity

Cath resonator & fil connector

Filament Connector Support

Filament Connector

Cathode Resonator

Cath Resonator on Tube

Cathode Resonator Insulators

Cathode Resonator Ass'y

Cath Resonator Capacitance

Output Coupling Loop

Tube Socket

Input Cavity Bottom Plate

Bottom Plate Ass'y

The pictures above are presented for
reference only. No further effort to document or develop this PA beyond this point is
anticipated or planned. Parts from this project will be "recycled" into a 33cm PA using
the excellent GS-15B tetrode.